72 thrower control-question for Mike10

I have a 72 thrower on a NH570.  Thing keeps getting stuck at a speed and won't go slower, only faster.  Open it all the way up and it helped for a while but yesterday that too was slow to respond.  Is there something to check or have I worn the joy switch out.  I have another control that I purchased for a second tractor but never used.  What I am really wondering is it in the control end or the receiving end?  thanks

72 thrower control-question for Mike10